The Western Subbasin is the second largest subbasin in the assessment area, with a catchment area of 219 square miles. This subbasin includes approximately 75 miles of the SF Eel River mainstem from Ohman Creek to its headwaters. There are approximately 387 miles of perennial (171 miles) and intermittent (140 miles) streams in this subbasin, and tributaries are located west of the Mainstem SF Eel River.

The Western Subbasin is located in the coastal fog belt, and streams are characterized by cool temperatures and relatively good flow in areas where there is limited residential development. Major tributaries in the Western Subbasin are Redwood Creek (near Redway), Sproul Creek, Indian Creek, and Hollow Tree Creek. Nearly 75% of the land use in the basin is timber harvest, and there is a very high density of roads, which increases fine sediment input throughout the Subbasin. Western Subbasin streams were generally suitable for salmonids, especially in upper Hollow Tree and Indian Creeks. Extensive diversion from Redwood Creek for illegal marijuana cultivation has been documented, and low flow, especially during late summer months, is a significant concern in many Western Subbasin streams.

Chinook salmon, coho salmon, and steelhead have been documented in more tributary miles in the Western Subbasin streams compared to other SF Eel River subbasin streams. Headwater streams near Branscomb have some of the highest quality habitat in the entire Basin, and some of these areas support relatively large populations of salmonids.